1. Field of the Invention
The present invention relates to an optical disk apparatus.
2. Description of Related Art
An optical disk, such as a photomagnetic disk, is adapted to a data recording method including a mark pit recording method and a mark edge recording method. Although the mark pit recording method does not encounter frequent commitment of errors because of direct recording of the NRZ code onto the optical disk ("1" indicates existence of a pit and "0" indicates no pit), it suffers from a problem in that the recording density cannot be raised because of a need to electrically distinguish portions "1" from each other in the case where portions "1" are formed successively.
On the other hand, the mark edge recording method is a method in which the NRZ code is converted into a code (for example, 1/7 byte code) in which portions "1" are not formed successively; and the edge position of the pit is made to correspond to "1" or the portion between edges is made to correspond to "0". The mark edge recording method enables the number of "0" to be determined in accordance with the length between "1" and "1". Since the foregoing method is able to raise the recording density by raising the frequency of the operation clocks, the mark edge recording method is in a trend to be employed in place of the mark pit recording method.
However, the mark edge recording method involving the structure such that the position of each "1" and the length between "1" and "1" correspond to data intended to be recorded and that intended to be reproduced has a necessity of accurately electrically reproducing the edge positions of the recorded pits as well as the necessity of accurately reflecting the state of configuration of "1" and "0" of data intended to be recorded onto the state where data is recorded on the medium.
In a case where the mark edge recording method is performed as shown in FIG. 1 (b) such that the portion of the medium between "1" and "1" is simply irradiated with a laser beam having predetermined power (level 1), a formed domain Ds is considerably different from an ideal domain Dp, as shown in FIG. 1 (a). Therefore, a pulse train method as shown in FIG. 2 is employed.
With the pulse train method, the level of the write laser output is maintained at a considerably low assist level (level 0) as shown in FIG. 2 (c) when no pit is formed. When pits are formed, the pulses are shifted to have a relatively high level (level 1) (about 2/3 of level 2 to be described later) capable of forming pits and a long (a period which is 3/2 of the operation clock shown in FIG. 2 (b)) period. Then, pulses having a higher level (level 2) and a shorter period (a period which is 1/2 of the operation clock) are used to write data. The above-mentioned method is able to form the domain Ds having a shape approximating the ideal domain Dp as shown in FIG. 2 (a). As a result, the alignment between data intended to be recorded and the domain for recording data can be maintained.
FIG. 3 is a diagram showing an example of an optical disk apparatus employing the mark edge recording method. Light reflected from an optical disk 1 comprising a photomagnetic disk is converted into an electric signal by a photodetector provided (not shown) for an optical head 11. The electric signal obtained by conversion is amplified by an amplifier 12, and then provided with a signal envelope by a low-pass filter 13. Thus, an analog signal as shown in FIG. 4 (a) is obtained Then, the peak level and the bottom level of the analog signal obtained from a predetermined track (or for a specific time) are confirmed, and then the obtained value is held in a peak/bottom holder 14. An intermediate value of the stored values is determined to be a slice level L.sub.0.
Then, a binary circuit 15 makes portions higher than the slice level L.sub.0 to be "1" and portions lower than the same to be "0", as shown in FIG. 4 (b) (that is, the analog signal is binary-coded). Then, as shown in FIGS. 4 (c) and 4 (d), dual data PDATA and NDATA denoting leading edge and trailing edges are obtained from the foregoing binary signal. By obtaining the logical sum of the dual data, reproduction data of recorded data can be obtained. Then, the obtained signal is, by a decoder 16, decoded to NRZ data required for a usual data process so as to be used.
Although the mark edge recording method is advantageous in realizing a high recording density as described above, there arises a necessity that the positions of "1" denoted by the recording signals and the intervals between "1" and "1" must be recorded and reproduced to correspond to the recording signals.
In a case where only optical disks of a predetermined type manufactured by a specific manufacturer are used, the optimum recording condition capable of preventing occurrence of errors can definitely be determined. However, the conditions of the optical disk, such as the material and the thickness of the magnetic film, are somewhat different among manufacturers. Therefore, even if the levels (levels 0 to 2) of the laser beam for use in recording data by the pulse train method are made to be the same, the state of the formed pits are somewhat different from one another in the case where the type of the optical disk is different. As a result, the optimum write condition cannot definitely be determined.
The pulse train method is required to adjust each of three levels of the laser beam in order to appropriately record data. Therefore, an appropriate write power must be set and therefore a long time is required to set the appropriate write power.
As a method of setting appropriate write power for an optical disk mounted on an optical disk apparatus, a test write method has been employed. The foregoing method comprises the steps of test-recording data on a predetermined test region of the mounted optical disk with a variety of powers; calculating the number of errors occurring in reproducing recorded data; and an optimum write power is determined in accordance with a result of the calculation above. However, since the reproducing condition is usually made to be constant in the foregoing process, a point, at which the number of errors can satisfactorily be decreased, cannot be detected precisely. Thus, there arises a problem in that a satisfactory accuracy cannot be realized in setting the appropriate write power.
A variety of methods of preventing reproduction errors by the reproducing unit have been suggested. For example, a method has been disclosed in, for example, Japanese Patent Application Laid-Open No. 3-91135, in which the amplifying ratio of the amplifying circuit and the amount of delay in the delay circuit are determined optimally in accordance with the state of reproduction. However, changes in the amplifying ratio and the amount of delay result in change in the reproduction waveform corresponding to FIG. 4 (a). Therefore, a simple attempt to decrease the number of errors by raising the amplifying ratio cannot be employed. Moreover, adjustment of the two factors, that is, the amount of delay and the amplifying ratio, causes the process to be complicated excessively.
When the slice level for use in the binary coding process is determined in the conventional structure, a middle point between the peak level and the bottom level of analog data reproduced from a specific track is obtained. In the case of data (1/7 data in this case) having domain portions and non-domain portions relatively uniformly distributed as shown in FIGS. 5A and 5B, the above-mentioned method of simply obtaining the middle point involves slight change in the slice levels La and Lb and capability of decreasing the number of errors. However, if a state in which the density of the domain formed portions is high is, as shown in FIG. 5C, rapidly changed to a state in which the density of the domain formed portions is low, the slice level Lc is sometimes changed. The change in the slice level causes the number of errors to be increased.